WO2019156461A1 - Positive electrode, and secondary battery comprising positive electrode - Google Patents

Abstract

The present invention relates to a positive electrode and a secondary battery, the positive electrode comprising a current collector and a positive electrode active material layer which is disposed on the current collector, wherein the positive electrode active material layer comprises a positive electrode active material, a binder and multi-walled carbon nanotubes, wherein the average length of the multi-walled carbon nanotubes is 1㎛ to 2㎛, and the standard deviation of the lengths of the multi-walled carbon nanotubes is 0.5㎛ or less.

Description

A positive electrode and a secondary battery including the positive electrode

Citation with Related Applications

This application claims the benefit of priority based on Korean Patent Application No. 10-2018-0015313, filed February 7, 2018, and all content disclosed in the literature of that Korean Patent Application is incorporated as part of this specification.

Field of technology

The present invention includes a current collector and a positive electrode active material layer disposed on the current collector, the positive electrode active material layer comprises a positive electrode active material, a binder, and multi-walled carbon nanotubes, the average length of the multi-walled carbon nanotubes 1 μm to 2 μm, and the standard deviation of the length of the multi-walled carbon nanotubes relates to a positive electrode and a secondary battery having 0.5 μm or less.

Recently, as technology development and demand for mobile devices increase, the demand for batteries as energy sources is rapidly increasing, and accordingly, researches on batteries capable of meeting various needs have been conducted. In particular, research on lithium secondary batteries having high energy density and excellent lifespan and cycle characteristics as a power source of such a device has been actively conducted.

The lithium secondary battery includes a positive electrode including a positive electrode active material capable of inserting / desorbing lithium ions, a negative electrode containing a negative electrode active material capable of inserting / removing lithium ions, and an electrode having a microporous separator interposed between the positive electrode and the negative electrode. It refers to a battery containing a nonaqueous electrolyte containing lithium ions in the assembly.

The positive electrode and / or the negative electrode may include a conductive material to improve conductivity. Conventionally, viscous conductive materials, such as carbon black, were mainly used. However, when the content of the conductive material is increased to improve conductivity, it is difficult to achieve a high energy density of the battery while relatively reducing the amount of the positive electrode active material or the negative electrode active material. Therefore, there is a demand for satisfying the power and durability required for a battery that is required even with a small amount of conductive material. In particular, in the case of the positive electrode, since the conductivity of the positive electrode active material itself is a low level, the problem appears more.

In order to solve this problem, a method of using a nano-sized conductive material, such as carbon nanotubes and carbon nanofibers, having a large specific surface area and allowing a large amount of conductive contact with a small amount is introduced. However, such a nano-size conductive material is difficult to be smoothly dispersed in the positive electrode slurry, so that the desired conductivity is difficult to be obtained unless the conductive material content in the positive electrode active material layer exceeds an appropriate level, for example, 1% by weight.

Accordingly, there is a demand for development of a cathode capable of improving dispersibility of a conductive material, ensuring conductivity despite a small content of a conductive material, and improving battery output and life characteristics.

An object of the present invention is to ensure the conductivity of the positive electrode even if the conductive material content is significantly reduced, the battery life characteristics can be improved, the content of the positive electrode active material can be increased, the positive electrode and the output characteristics of the battery can be improved It is to provide a secondary battery including the same.

The present invention includes a current collector and a positive electrode active material layer disposed on the current collector, the positive electrode active material layer comprises a positive electrode active material, a binder, and multi-walled carbon nanotubes, the average length of the multi-walled carbon nanotubes 1 μm to 2 μm, and a standard deviation of the length of the multi-walled carbon nanotubes provides a positive electrode of 0.5 μm or less.

In addition, the present invention is the positive electrode; cathode; A separator interposed between the anode and the cathode; And it provides a secondary battery comprising an electrolyte.

According to the present invention, a multi-walled carbon nanotube is used as a conductive material, and the multi-walled carbon nanotube has a standard deviation of an average length of an appropriate level and an appropriate level. Accordingly, the dispersion of the multi-walled carbon nanotubes in the conductive material dispersion and the positive electrode slurry can be made uniform, and the positive electrode active materials can be electrically connected smoothly by the multi-walled carbon nanotubes in the prepared positive electrode. Accordingly, the life characteristics of the battery can be improved. In addition, since the dispersibility of the multi-walled carbon nanotubes is improved, even when the multi-walled carbon nanotubes are used in a small amount, the conductivity of the positive electrode can be ensured, so that the content of the positive electrode active material can be relatively increased. Accordingly, output characteristics of the manufactured secondary battery can be improved.

1 is a graph showing the measurement of the length of the multi-walled carbon nanotubes contained in the anode used in Example 1 of the present invention.

Figure 2 is a graph showing the measurement of the length of the multi-walled carbon nanotubes contained in the anode used in Example 2 of the present invention.

Figure 3 is a graph showing the measurement of the length of the multi-walled carbon nanotubes contained in the anode used in Comparative Example 1 of the present invention.

Figure 4 is a graph showing the measurement of the length of the multi-walled carbon nanotubes contained in the anode used in Comparative Example 2 of the present invention.

5 is a graph showing an increase in discharge capacity and battery resistance with cycles for the batteries according to Examples 1 and 2 and Comparative Examples 1 and 2. FIG.

Hereinafter, the present invention will be described in more detail to aid in understanding the present invention. At this time, the terms or words used in the present specification and claims should not be construed as being limited to the ordinary or dictionary meanings, and the inventors appropriately define the concept of terms in order to explain their invention in the best way. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that it can.

<Anode>

A positive electrode according to an embodiment of the present invention, a current collector and a positive electrode active material layer disposed on the current collector, the positive electrode active material layer includes a positive electrode active material, a binder, and multi-walled carbon nanotubes, the multi The average length of the wall carbon nanotubes is 1 μm to 2 μm, and the standard deviation of the length of the multi-walled carbon nanotubes may be 0.5 μm or less.

The current collector may be any conductive material without causing chemical change in the battery, and is not particularly limited. For example, the current collector may be copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or a surface treated with carbon, nickel, titanium, silver, or the like on the surface of aluminum or stainless steel. Specifically, a transition metal that adsorbs carbon such as copper and nickel can be used as the current collector.

The positive electrode active material layer may be disposed on one surface or both surfaces of the current collector. The cathode active material layer may include a cathode active material, a binder, and multi-walled carbon nanotubes.

The cathode active material may be the same as the cathode active material included in the cathode slurry of the above-described embodiment. Specifically, the cathode active material may be a cathode active material that is commonly used. Specifically, the cathode active material may be a layered compound such as lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), or a compound substituted with one or more transition metals; Lithium manganese oxides such as Li _{1 + y 1} Mn _{2-y 1} O ₄ (0 ≦ _{y 1} ≦ 0.33), LiMnO ₃ , LiMn ₂ O ₃ , LiMnO ₂ ; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , V ₂ O ₅ , Cu ₂ V ₂ O _7, and the like; Ni-site type lithium nickel oxide represented by the formula LiNi _1-y2 M1 _y2 O ₂ , wherein M1 is Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and satisfies 0.01 ≦ y2 ≦ 0.3; Formula LiMn _2-y3 M2 _y3 O ₂ , wherein M2 is Co, Ni, Fe, Cr, Zn or Ta and satisfies 0.01 ≦ y3 ≦ 0.1 or Li ₂ Mn ₃ M ₃ O ₈ , wherein M ₃ is Fe, Lithium manganese composite oxide represented by Co, Ni, Cu, or Zn; LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with alkaline earth metal ions, etc. may be mentioned, but is not limited thereto. Specifically, the cathode active material may be Li [Ni _0.6 Mn _0.2 Co _0.2 ] O ₂ .

The cathode active material may be included in an amount of 96% by weight to 99% by weight based on the total weight of the cathode active material layer, and specifically, may be included in an amount of 97% by weight to 98.5% by weight. When the above range is satisfied, the battery life can be improved, but the multi-walled carbon nanotubes and the binder content do not decrease excessively, so the battery life characteristics can be maintained.

The binder may be the same as the binder included in the positive electrode slurry of the above-described embodiment. Specifically, the binder is polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (polyvinylidenefluoride), polyacrylonitrile (polyacrylonitrile), polymethylmethacrylate (polymethylmethacrylate) And at least one selected from the group consisting of substances in which hydrogen is substituted with Li, Na, or Ca, and may also include various copolymers thereof. It may be a polyvinylidene fluoride containing a functional group, such as an ether group.

The multi-walled carbon nanotubes may refer to carbon nanotubes having two or more graphene layers disposed side by side with respect to the axis of the carbon nanotubes. That is, the number of walls may mean carbon nanotubes of 2 or more. In the present invention, carbon nanotubes having a preferred length and standard deviation of the length may be formed during the dispersion of the conductive material dispersion.

The average length of the multi-walled carbon nanotubes may be 1 μm to 2 μm, specifically 1.1 μm to 1.4 μm, and more specifically 1.1 μm to 1.2 μm. When the average length of the multi-walled carbon nanotubes is less than 1㎛, a plurality of too short multi-walled carbon nanotubes are present, the electrical connection between the positive electrode active material can not be made smoothly, the output of the battery is reduced. On the other hand, when the average length of the multi-walled carbon nanotubes exceeds 2㎛, a large number of excessively long multi-walled carbon nanotubes are present, so that the multi-walled carbon nanotubes can be easily entangled with each other in the conductive dispersion and the positive electrode slurry. have. Accordingly, since the multi-walled carbon nanotubes are not uniformly distributed in the positive electrode active material layer, the conductivity of the positive electrode active material layer is lowered. Accordingly, the output and lifespan characteristics of the battery may be lowered.

On the other hand, the average length of more than 2㎛ means that the length of the multi-walled carbon nanotubes is not short enough, which may be due to the lack of a particle size distribution control process such as milling. Therefore, the average length of more than 2㎛ the standard deviation of the length of the multi-walled carbon nanotubes can not only be large, specifically exceed 0.5㎛.

The standard deviation of the length of the multi-walled carbon nanotubes may be 0.5 μm or less, and specifically 0.3 μm to 0.5 μm. If the standard deviation of the length of the multi-walled carbon nanotubes exceeds 0.5㎛, the length difference between the multi-walled carbon nanotubes is large, the conductivity in the positive electrode active material layer is not formed to a uniform degree. As a result, battery output and lifespan characteristics may be degraded. Furthermore, the excessively large standard deviation may be due to the insufficient process of improving the particle size of the multi-walled carbon nanotubes such as milling in the manufacture of the conductive material dispersion, and thus the separation of the multi-walled carbon nanotubes in the conductive material dispersion and the positive electrode active material layer. Since the acidity is lowered, the battery output and lifespan characteristics are further lowered. In addition, when the standard deviation is too large, the viscosity of the conductive material dispersion and the positive electrode slurry is excessively increased. Accordingly, when the cathode active material layer is formed, it is difficult to smoothly apply the cathode slurry, and thus the output and life characteristics of the battery may be further reduced.

The length of the multi-walled carbon nanotubes may be 0.5 μm to 3.0 μm, and specifically 0.7 μm to 2.5 μm. Here, the length means not the average length described above, but the length of each of the observed multi-walled carbon nanotubes. When satisfying the above range, while maintaining the electrical connection between the positive electrode active material at an appropriate level, the dispersion of the multi-walled carbon nanotubes in the conductive material dispersion and the positive electrode slurry can be made uniform. Even in the case where a small amount of multi-walled carbon nanotubes having a length exceeding 3.0 μm is included in the positive electrode active material layer, aggregation of the multi-walled carbon nanotubes occurs by the long multi-walled carbon nanotubes, so that the inside of the positive electrode active material layer Multi-walled carbon nanotubes are uniformly dispersed and difficult to exist. Therefore, the output and lifespan characteristics of the battery may be lowered. In addition, when the long multi-walled carbon nanotubes are present, since the agglomeration occurs, the viscosity of the conductive material dispersion is increased, so that the processability is lowered during fabrication of the cathode active material layer. Dispersibility of the wall carbon nanotubes may be further lowered.

With respect to the multi-walled carbon nanotubes included in the positive electrode active material layer, the average length, standard deviation of the length of the multi-walled carbon nanotubes described above may be measured by the following method. First, a predetermined amount of the positive electrode active material layer is diluted in an NMP solution amounting to several tens of times of weight, and then, materials constituting the positive electrode active material layer are separated through ultrasonic waves. Thereafter, a part of the upper layer portion of the solution is extracted, and further diluted with an NMP solution weighing tens of times the amount of extraction. After that, the length of a plurality of, for example, 30 or 25 multi-walled carbon nanotubes was measured by scanning electron microscopy (SEM), respectively, and their average and standard deviation were calculated to obtain the multi-walled carbon nanotubes. We can derive the mean length, standard deviation of length, and length.

The multi-walled carbon nanotubes may be included in an amount of 0.1 wt% to 1 wt% based on the total weight of the cathode active material layer, specifically, 0.2 wt% to 0.9 wt%, and more specifically 0.2 wt% to 0.7 wt%. It may be included in weight percent. When the above range is satisfied, conductivity of the positive electrode active material layer may be secured.

The inclusion of the multi-walled carbon nanotubes in an amount of 1% by weight or less, in particular 0.7% by weight or less, is hardly achieved only by carbon nanotubes having general physical properties. Specifically, in order to increase the content of the positive electrode active material, if the content of a relatively common carbon nanotube is reduced to a level of 1% by weight or less, the electrical connection between the positive electrode active materials can never be made smoothly the output characteristics of the secondary battery manufactured This is inevitably greatly reduced. In addition, since the cathode active materials may not be smoothly supported by the carbon nanotubes, the cathode active materials may be detached from the cathode active material layer, or the structure of the cathode active material layer may be easily collapsed, thereby deteriorating mechanical stability of the cathode. Accordingly, the cycle characteristics of the manufactured secondary battery inevitably deteriorate.

However, in the present invention, since the positive electrode active material layer contains the multi-walled carbon nanotubes having physical properties such as a proper average length and standard deviation of the length in a uniformly distributed state, the content of the multi-walled carbon nanotubes is 1% by weight. Even if it is included below, the electrical connection between the positive electrode active material is maintained, the mechanical stability of the positive electrode active material layer can be secured, the output and life characteristics of the battery can be improved. In addition, since the content of the multi-walled carbon nanotubes is maintained at a level of less than 1% by weight, the cathode active material layer may include a cathode active material having a higher content, and thus the output of the manufactured battery may be further improved. Can be.

The loading amount of the positive electrode active material layer may be 15 mg / cm ² to 40 mg / cm ² , and specifically 20 mg / cm ² to 30 mg / cm ² . When satisfying the above range, while securing the energy density of the positive electrode, the positive electrode thickness may not increase excessively. In addition, a problem may not occur in processability when applying the positive electrode slurry.

<Method of manufacturing anode>

Method for producing a positive electrode according to another embodiment of the present invention, preparing a conductive material dispersion; Forming a positive electrode slurry comprising a conductive material dispersion, a positive electrode active material, a binder, and a solvent; And applying and drying the positive electrode slurry on a current collector, wherein the conductive material dispersion includes a multiwall carbon nanotube, a dispersant, and a dispersion medium, and the average length of the multiwall carbon nanotube is 1 μm to 1 μm. 2 μm, and the standard deviation of the length of the multi-walled carbon nanotubes may be 0.5 μm or less.

The preparing of the conductive material dispersion may include forming a mixture by mixing bundle-type multiwall carbon nanotubes, a dispersant, and a dispersion medium, and adjusting the particle size distribution of the bundle multiwall carbon nanotubes. It may include doing.

The dispersant may be at least one selected from the group consisting of hydrogenated nitrile butadiene rubber (H-NBR), polyvinylpyrrolidone (PVP), and carboxymethylcellulose (CMC).

The dispersion medium may be at least one of N-methyl-2-pyrrolidone (NMP) and water.

In the bundled multi-walled carbon nanotube, the bundle includes a bundle or a rope in which a plurality of carbon nanotube units are arranged or intertwined side by side in substantially the same orientation in the longitudinal direction of the unit. Refers to the secondary shape of the form. In the case of the bundled multi-walled carbon nanotubes, the carbon nanotubes have a partially agglomerated form, and their lengths are excessively varied, so that if they are used directly as a conductive material without adjusting their shape and length, the bundled multi-walled carbon nanotubes may be bundled in the positive electrode active material layer. Wall carbon nanotubes are uniformly dispersed and difficult to exist, and it is difficult to secure a conductive path. Therefore, after mixing the bundle-type multi-walled carbon nanotubes, a dispersant, and a dispersion medium, there is a need for a process for controlling the particle size distribution, that is, the shape and length of the bundled multi-walled carbon nanotubes.

In particular, the bundle-type multi-walled carbon nanotubes immediately after the synthesis is completed are randomly aggregated carbon nanotubes of 5 ㎛ to 50 ㎛ to have a particle size of several tens of ㎛. Typically, pellets are manufactured from the bundled multi-walled carbon nanotubes for ease of handling such as transport, storage, and feeding. Accordingly, in order to use the anode as a conductive material, milling operations are required in which the pellets are decomposed to separate bundle-type carbon nanotubes from each other and have a uniform length.

The particle size distribution may be adjusted by milling, ultrasonication, or the like, and preferably by milling. The milling may be performed by a ball mill, spike mill, bead mill, basket mill, attrition mill, or the like. Can be advanced by a spike mill.

The spike mill may proceed in the following manner. The spike mill is operated while injecting a mixture including the bundled carbon nanotubes, a dispersant, and a dispersion medium into a spike mill device filled with beads. At this time, the rotor inside the device rotates, and this rotational force imparts kinetic energy to the beads, resulting in dispersion of the bundled carbon nanotubes in the mixture. The mixture is then discharged through the outlet at a particular discharge rate. This process may be performed under specific conditions to form multi-walled carbon nanotubes included in the anode of the present invention. In particular, the size of the beads, the filling amount of the beads, the discharge rate of the mixture, the number of milling is a major condition, the conductive material dispersion used in the present invention can be formed by a suitable combination thereof. In other words, the combination of each condition as well as the range of each of the above conditions must be properly satisfied.

The size of the beads may be 0.5mm to 2mm, specifically 0.6mm to 1mm, may be more specifically 0.6mm to 0.75mm. When the size of the beads exceeds 2mm, the shear force generated by the beads is not enough, the dispersion and particle size distribution of the multi-walled carbon nanotubes may not reach the desired level. In addition, when the size of the bead is less than 0.5mm, the beads as well as the mixture is discharged together to the outlet, the dispersion capacity of the spike mill is not kept constant, and the process of separating the beads from the discharged result is required separately There is.

The filling amount of the beads may be 50% to 90%, specifically, 65% to 80%. When the filling amount of the beads is more than 90%, the pressure inside the spike mill apparatus is greatly increased, making continuous spike mill use difficult. If the filling amount of the beads is less than 50%, proper kinetic energy required for dispersion is difficult to form.

The discharge rate of the mixture may be 1 kg / min to 5 kg / min, specifically 2 kg / min to 4 kg / min.

The number of milling means the number of times the mixture is introduced into the container. The number of milling may be 2 to 3 times.

The conductive material dispersion may have a viscosity of 10,000 cps to 30,000 cps at 30 ° C. to 50 ° C., specifically 15,000 cps to 25,000 cps. When satisfying the above range, it is easy to add during the production of the positive electrode slurry. In addition, satisfying the viscosity means that the dispersion of the multi-walled carbon nanotubes is smoothly performed and the particle size distribution satisfies a desirable level.

In the forming of the positive electrode slurry, the positive electrode slurry may include the conductive material dispersion, a positive electrode active material, a binder, and a solvent.

The positive electrode active material, the binder, the multi-walled carbon nanotubes, and the current collector are the same as the positive electrode active material, the binder, and the multi-walled carbon nanotubes included in the positive electrode of the above-described embodiment, and thus description thereof is omitted. On the other hand, the average length of the multi-walled carbon nanotubes contained on the conductive material dispersion, the standard deviation of the length, the length can be maintained the same in the positive electrode active material layer.

The solvent is dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methyl-2-pyrrolidone (N-Methyl-2-Pyrrolidone, NMP), polyvinylidene fluoride-hexafluoro It may be at least one selected from the group consisting of an aqueous solution of a low temperature propylene copolymer (PVDF-co-HFP), an aqueous solution of polyvinylidenefluoride, acetone and water. For example, the solvent may be NMP.

Solid content of the positive electrode slurry may be 60% by weight to 80% by weight based on the total weight of the positive electrode slurry, specifically, may be 65% by weight to 75% by weight. When satisfying the above range, while maintaining the viscosity enough to apply (coating) the positive electrode slurry to the current collector, there is an advantage that the positive electrode slurry is easy to dry. At this time, the viscosity of the positive electrode slurry is preferably 5,000cps to 25,000cps.

Since the positive electrode active material layer is manufactured by drying the positive electrode slurry to remove the solvent, the content of each of the positive electrode active material, the binder, and the multi-walled carbon nanotubes included in the solid content is based on the total weight of the solid content of the positive electrode slurry. It is equivalent to the numerical value based on the total weight of the positive electrode active material layer included in the positive electrode of one embodiment.

In the coating and drying step, the coating and drying may be to apply and dry the current collector to which the positive electrode slurry is applied at a rate of 4m / min to 80m / min at a temperature of 100 ℃ to 180 ℃. In addition, roll drying may be performed to adjust the thickness of the anode after drying, and further drying may be performed to remove residual moisture of the anode after rolling.

<Secondary battery>

A secondary battery according to another embodiment of the present invention may include a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. Here, since the anode is the same as the anode of the above-described embodiment, a description thereof will be omitted.

The negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on one or both surfaces of the negative electrode current collector.

The negative electrode current collector may be any one having conductivity without causing chemical change in the battery, and is not particularly limited. For example, the negative electrode current collector may be copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or a surface treated with carbon, nickel, titanium, silver, or the like on the surface of aluminum or stainless steel. . Specifically, a transition metal that adsorbs carbon such as copper and nickel can be used as the current collector.

The negative electrode active material layer may include a negative electrode active material, a negative electrode conductive material, and a negative electrode binder.

The negative electrode active material may be graphite-based active material particles or silicon-based active material particles. The graphite-based active material particles may be used at least one selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fibers and graphitized mesocarbon microbeads, and in particular, when using artificial graphite, the rate characteristics may be improved. . The silicon-based active material particles are Si, SiO _x (0 <x <2), Si-C composite and Si-Y alloy (where Y is an alkali metal, alkaline earth metal, transition metal, group 13 element, group 14 element, rare earth element And it is an element selected from the group consisting of a combination thereof) may be used one or more selected from the group consisting of, in particular, when using Si can derive a high capacity of the battery.

The negative electrode binder may be polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, Polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), Sulfonated EPDM, styrene butadiene rubber (SBR), fluorine rubber, poly acrylic acid (poly acrylic acid) and may include at least one selected from the group consisting of substances substituted with hydrogen, such as Li, Na or Ca, And also various copolymers thereof.

The negative electrode conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, farnes black, lamp black and thermal black; Conductive fibers such as carbon fibers and metal fibers; Conductive tubes such as carbon nanotubes; Metal powders such as fluorocarbon, aluminum and nickel powders; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The separator is to separate the negative electrode and the positive electrode and to provide a passage for the movement of lithium ions, and can be used without any particular limitation as long as the separator is used as a separator in a secondary battery. It is desirable to be excellent. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer or the like Laminate structures of two or more layers may be used. In addition, conventional porous nonwoven fabrics such as nonwoven fabrics made of high melting point glass fibers, polyethylene terephthalate fibers and the like may be used. In addition, a coated separator including a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, and may be optionally used as a single layer or a multilayer structure.

The electrolyte may include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, and the like, which can be used in manufacturing a lithium secondary battery, but are not limited thereto.

Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

As the non-aqueous organic solvent, for example, N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butylo lactone, 1,2-dime Methoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, formamide, dimethylformamide, dioxoron, acetonitrile, nitromethane, methyl formate, Methyl acetate, phosphate triester, trimethoxy methane, dioxorone derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, pyrion An aprotic organic solvent such as methyl acid or ethyl propionate can be used.

In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates among the carbonate-based organic solvents, may be preferably used as high-viscosity organic solvents because they have high dielectric constants to dissociate lithium salts well, such as dimethyl carbonate and diethyl carbonate. When the same low viscosity, low dielectric constant linear carbonate is mixed and used in an appropriate ratio, an electrolyte having a high electrical conductivity can be made, and thus it can be more preferably used.

The metal salt may be a lithium salt, the lithium salt is a material that is readily soluble in the non-aqueous electrolyte, for example, is in the lithium salt anion ^{F -, Cl -, I -} , NO 3 -, N (CN _{^{) 2 -, BF 4 -,}} ClO 4 -, PF 6 -, (CF 3) 2 PF 4 -, (CF 3) 3 PF 3 -, (CF 3) 4 PF 2 -, (CF 3) 5 PF - ^{_{, (CF 3) 6 P -}} , CF 3 SO 3 -, CF 3 CF 2 SO 3 -, (CF 3 SO 2) 2 N -, (FSO 2) 2 N -, CF 3 CF 2 (CF 3) 2 _{^{CO -, (CF 3 SO 2}} ) 2 CH -, (SF 5) 3 C -, (CF 3 SO 2) 3 C -, CF 3 (CF 2) 7 SO 3 -, CF 3 CO 2 -, CH 3 CO ₂ ^-, SCN ^- may be used one selected from the group consisting of ^- and _{(CF 3 CF 2 SO 2)} 2 N.

In addition to the electrolyte components, the electrolyte includes, for example, haloalkylene carbonate-based compounds such as difluoro ethylene carbonate, pyridine, tri, etc. for the purpose of improving battery life characteristics, reducing battery capacity, and improving discharge capacity of the battery. Ethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N, N-substituted imida One or more additives such as zolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol or aluminum trichloride may be included.

According to another embodiment of the present invention, a battery module including the secondary battery as a unit cell and a battery pack including the same are provided. Since the battery module and the battery pack include the secondary battery having high capacity, high rate characteristics, and cycle characteristics, a medium-large device selected from the group consisting of an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage system It can be used as a power source.

Examples and Comparative Examples

Example 1 Fabrication of Cells

(1) Preparation of Dispersion of Conductive Material

Bundled multi-walled carbon nanotubes in pellet form of several cm size, H-NBR as a dispersant, and NMP as a dispersion medium were mixed at a weight ratio of 4: 0.8: 95.2 to form a mixture. The mixture was dispensed into a spike mill filled with 80% of beads 0.65 mm in size and discharged at a discharge rate of 2 kg / min. Such a process was carried out twice to prepare a conductive material dispersion including a multi-walled carbon nanotube having a controlled particle size distribution.

(2) production of anode

Li (Ni _0.6 Mn _0.2 Co _0.2 ) O ₂ was used as the positive electrode active material and PVdF was used as the binder. The positive electrode active material, the conductive material dispersion, and the NMP were mixed to prepare a positive electrode slurry having a solid content of 72% and a weight ratio of the positive electrode active material, the binder, the dispersant, and the multi-walled carbon nanotubes of 98: 1.52: 0.08: 0.4.

The positive electrode slurry was applied and dried to a positive electrode current collector (Al) having a thickness of 20 μm so that the solid content weight (loading amount) was 21 mg / cm ² . Thereafter, the current collector on which the positive electrode slurry was placed was rolled by a roll rolling method to adjust the overall thickness to 77 μm. Thereafter, the current collector was dried in a vacuum oven at 110 ° C. for 6 hours to prepare a positive electrode.

(3) production of batteries

A negative electrode slurry was prepared by mixing artificial graphite as a negative electrode active material, carbon black as a negative electrode conductive material, styrene butadiene rubber (SBR) as a negative electrode binder, and distilled water at a weight ratio of 96.1: 0.5: 2.3: 1.1, respectively. The prepared slurry was applied and dried to a negative electrode current collector (Cu) having a thickness of 20 μm so that the weight (loading amount) was 10 mg / cm ² . Thereafter, the current collector on which the positive electrode slurry was formed was rolled by a roll rolling method, and the final thickness (current collector + active material layer) was adjusted to be 80 μm. Thereafter, the current collector was dried in a vacuum oven at 110 ° C. for 6 hours to prepare a negative electrode.

The prepared negative electrode and the positive electrode and the porous polyethylene separator were assembled using a stacking method, and the assembled battery was an electrolyte solution (ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 1/2 (volume ratio), lithium hexa). Floro phosphate (LiPF ₆ 1 mol) was injected to prepare a lithium secondary battery.

Example 2: Preparation of Cells

In preparing the conductive material dispersion, the conductive material dispersion, the positive electrode, and the battery were prepared in the same manner as in Example 1 except that the bead size was changed to 1 mm.

Comparative Example 1: Preparation of Battery

In preparing the conductive material dispersion, the conductive material dispersion, the positive electrode, and the battery were prepared in the same manner as in Example 1 except that the bead size was changed to 1 mm and the recovery was performed once.

Comparative Example 2: Preparation of Battery

In preparing the conductive material dispersion, the conductive material dispersion, the positive electrode, and the battery were manufactured in the same manner as in Example 1 except that the number of millings was changed to four times.

The physical properties of the multi-walled carbon nanotubes present in the conductive material dispersion formed in the battery manufacturing process of Examples 1 and 2 and Comparative Examples 1 and 2 were measured by the following method, respectively, and then, respectively, FIG. 1 (used in Example 1). Multi-walled carbon nanotubes), Figure 2 (multi-walled carbon nanotubes used in Example 2), Figure 3 (multi-walled carbon nanotubes used in Comparative Example 1), Figure 4 (multiple used in Comparative Example 2 Wall carbon nanotubes) and shown in Table 1.

Specifically, NMP was further added to each conductive material dispersion to prepare a dilute solution having 0.005 wt% of the multi-walled carbon nanotubes based on the total weight of the solution. The diluted solution was dropped on a silicon wafer and dried to prepare each sample, and the sample was observed by SEM. At this time, after measuring the length of the observed 25 multi-walled carbon nanotubes, the average length, standard deviation of the length, the maximum length, the minimum length was derived.

	Properties of Multi-Walled Carbon Nanotubes in Conductive Dispersions
	Average length (㎛)	Standard Deviation of Length (μm)	Max length (㎛)	Length (μm)
Example 1	1.26	0.45	2.68	0.59
Example 2	1.16	0.50	2.38	0.50
Comparative Example 1	1.56	0.81	4.50	0.50
Comparative Example 2	0.78	0.46	1.93	0.05

Experimental Example 1 Evaluation of Discharge Capacity and Battery Resistance According to Charge and Discharge Cycles

After charging and discharging each of the secondary batteries manufactured in Examples 1 and 2 and Comparative Examples 1 and 2, the discharge capacity and the battery resistance were evaluated, and are shown in the graphs and Table 2 of FIG. 5.

One cycle and two cycles were charged and discharged at 0.1C, and charging and discharging was performed at 0.5C from three cycles to 200 cycles.

Charging conditions: CC (constant current) / CV (constant voltage) (4.2V / 0.05C current cut-off)

Discharge condition: CC (constant current) condition 2.7 V

In the graph of FIG. 5, the left y-axis represents the discharge capacity (%) of each cycle when the discharge capacity of one cycle is 100%, and the right y-axis represents the increase ratio of the battery resistance of each cycle to the battery resistance of one cycle. (%).

The battery resistance was calculated from the voltage drop that occurs when a current of 3C was applied for 30 seconds while charging 50% of the total battery capacity.

	Discharge capacity (%) of 200 cycles	% Of battery resistance increase in 200 cycles
Example 1	95.03	20
Example 2	96.4	16
Comparative Example 1	85.24	185
Comparative Example 2	90.41	96

5 and Table 2, the average length of the multi-walled carbon nanotubes included in the positive electrode active material layer satisfies 1㎛ 2㎛ and the standard deviation of the length of the battery of Examples 1 and 2 In this case, it can be seen that the capacity retention rate is higher than that of Comparative Example 1 and Comparative Example 2, and the battery resistance increases much less.

Specifically, in Comparative Example 1, the average length of the multi-walled carbon nanotubes satisfies 1 μm to 2 μm, but the standard deviation of the lengths is 0.81 μm and has a value exceeding 0.5 μm. That is, Comparative Example 1 means that the multi-walled carbon nanotubes having an excessively varying length are included, and the coagulation between the multi-walled carbon nanotubes is generated by the multi-walled carbon nanotubes of some excessively long lengths, thereby forming the positive electrode active material layer. It seems that the multi-walled carbon nanotubes are not uniformly dispersed at. Thereby, it turns out that the capacity retention rate and resistance characteristic of a battery fall.

In the case of Comparative Example 2, the standard deviation of the length is less than 0.5㎛, but the average length of the multi-walled carbon nanotube is 0.78㎛, does not satisfy the range of 1㎛ to 2㎛. That is, since the length of the multi-walled carbon nanotubes is too short, the electrical connection between the positive electrode active material is not formed smoothly. It can be seen that the capacity retention rate and the resistance characteristics of the battery are thereby deteriorated.

Claims (7)

1. A current collector and a positive electrode active material layer disposed on the current collector;

   The positive electrode active material layer includes a positive electrode active material, a binder, and multi-walled carbon nanotubes,

   The average length of the multi-walled carbon nanotubes is 1㎛ to 2㎛,

   The standard deviation of the multi-walled carbon nanotube length is 0.5㎛ or less.
2. The method according to claim 1,

   A length of the multi-walled carbon nanotubes is 0.5㎛ 3.0㎛.
3. The method according to claim 1,

   The multi-walled carbon nanotubes include 0.1 wt% to 1 wt% based on the total weight of the cathode active material layer.
4. The method according to claim 1,

   The multi-walled carbon nanotubes include 0.2 wt% to 0.7 wt% based on the total weight of the cathode active material layer.
5. The method according to claim 1,

   The positive electrode active material is a positive electrode containing 96% by weight to 99% by weight based on the total weight of the positive electrode active material layer.
6. The method according to claim 1,

   A loading amount of the positive electrode active material layer is 15 mg / cm ² to 40 mg / cm ² .
7. The anode of any one of claims 1 to 6;

   cathode;

   A separator interposed between the anode and the cathode; And

   Secondary battery comprising an electrolyte.

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